Alitalia was forced to cancel more than 350 flights, affecting 30,000 passengers, when flight attendants walked off their jobs May 3.

Unions announced Tuesday in a joint statement that they would strike again for 24 hours on May 21.

Flight attendants at Alitalia are in contract renewal negotiations. The ailing Italian airline, which reported widening pretax losses in 2006, has been battered by a series of strikes as well as high fuel costs and competition from low-cost airlines.

The Italian government, which controls a 49.9 percent share of Alitalia, hopes to sell at least a 39.9 percent share to a buyer with a turnaround plan.